In general, multiple input multiple output (MIMO) operation increases the capacity of a radio link through the use of multiple transmit antennas and multiple receive antennas. MIMO exploits multipath propagation to increase the capacity of the radio link. MIMO has proven to be effective at increasing the capacity of the radio link and has been accepted into a variety of technical standards, including: WiFi or Wireless LAN, such as IEEE 802.11n and IEEE 802.11ac; Evolved High-Speed Packet Access (HSPA+); Worldwide Interoperability for Microwave Access (WiMAX); and Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Advanced.
Increasing the number of transmit antennas and receive antennas from a relatively small number (on the order of 10 or fewer) to a significantly larger number (on the order of 100, 1000, 10000, or more) can lead to even greater increases in the capacity of the radio link. Such MIMO communications systems are referred to as large-scale MIMO communications systems.
Channel estimation is a complex and time intensive operation in which the characteristics of a communication channel are estimated for the purpose of reducing transmission error rates and improving efficiency. Under the multi-path model, channel estimation is performed for every multi-path at each antenna and involves individually receiving reference signals transmitted over each multi-path for each transmitter-receiver pair. Therefore, at a MIMO communications device, such as a large scale MIMO communications device, the number of channel estimations can be very large. For example, in a 10000 antenna MIMO communications device with 3 multi-paths, there will be 30000 channel estimations even for a user equipped with a single antenna. Additionally, since the user may move or the environment changes, the channel estimations may be repeated regularly to maintain effective models. Therefore, the signaling overhead and the computational complexity significantly impacts overall communications system performance in a negative way.